Renamed RunningTask.onNext() to RunningTask.send()
 * Added a method `RunningTask.send(byte[])`
 * Deprecated `RunningTask.onNext(bye[])`
 * Added the implementation in `RunningTaskImpl`
 * Updated all examples to ose `send(bye[]`
Merge branch 'master' into weimer_789
Merge branch 'master' of github.com:Microsoft-CISL/REEF into YingdaReef
Added the JCIP annotations as a dependency
  * Added the Java Concurrency in Practice annotations as a dependency
  * Annotated `Optional` as an example.
Driver Singleton cleanup
  * Removed the named parameter `DriverSingletons` as it wasn't safe with Tang
  * Added the service event handlers to the class `DriverSingletons` as a dependency instead to make sure they are only instantiated once per Driver.
Merge pull request #800 from Microsoft-CISL/weimer_singletons

Driver Singleton cleanup
Merge pull request #799 from Microsoft-CISL/weimer_jcip

Add the JCIP annotations as a dependency
Merge branch 'master' of github.com:Microsoft-CISL/REEF into YingdaReef
minor fixes in YarnContainerManager - code cleanup, simplify
synchronization, use proper logging etc.